Correct way to render crepuscular rays in cycles?
This is really a tricky one.

First off, world surface shader and sun lamps dont work for world volume shading for now (as stated in the manual). Also, if you use object volumes the camera can not be inside the volume. But you can make a cube bubble around the camera and parent it to the camera. E.g use a "Boolean Modifier" with the bubble (intersect) to cut it off. Of course this only works when your bubble sticks out of the volume but you can extrude it in any direction that doesnt point forward anyway.  



Here is an example of the node setup for a volume:
volume scatter shader and volume absorbtion shader as input to an add shader. Thats it. 



For the sun lamp use simple emission with strength of e.g. 9 to get deeper shading for the rays. Also helpful is to set the sampling size for the shadow to 0 to get crisp lines. Background can be set to a Sky Texture.

To keep render settings simple volume scattering events can be set to 0. For the Volume Sampling it might be necessary to tweak the Max Steps (e.g. 2048) and Step Size (e.g. 0.04).



This is what it would look like: 



To get more crepuscular rays (like in the image you provided) you could add some shadow shapes between the sun and the volume (acting as clouds). Of course this example looks horrible but you get the idea.